A family that has an abnormal von Willebrand factor (vWf) with a defective binding site for factor VIII has been studied, and the genetic defect has been identified. The binding defect was initially evaluated by assessing the ability of the patient's vWf to bind purified factor VIII. Subsequently, DNA was purified from peripheral blood leukocytes, specific regions of the vWf gene were amplified by polymerase chain reaction (PCR), and direct sequencing of the DNA was carried out. A transition of nucleotide 2451 (Tto A) was found, which results in the substitution of Gln for His at amino acid 54 in the mature vWf subunit. We recently used a PCR mutagenesis technique to insert the mutation into cloned DNA and are in the process of cloning this DNA and expressing the abnormal protein. The latter will be used in our binding assay for factor VIII to assess its binding capacity. A second family with a different variant of von Willebrand's disease and a vWf with an abnormal affinity for binding to platelets has been identified. Studies of the binding region of the vWf for platelets have not yielded any abnormalities to date.